Various diagnostic devices have been devised for providing useful timing and cylinder pressure information from fuel-injected diesel engines. For example, U.S. Pat. No. 4,036,050 of one of the applicants, Joseph Dooley, and Edward Yelke discloses various transducers incorporating piezoelectric material mounted so as to respond to pressures created within the holding clamps, dogs or retaining bolts of an injection nozzle assembly of a fuel-injected engine to provide electrical signals which are generally representative of the pressure variations within the cylinder. These devices are generally limited to engines using some sort of bolts, clamps, or dogs for holding the cylinder injection nozzle in constant engagement with its access port to the cylinder. Many engine types do not utilize such nozzle retaining apparatus. Other transducers have been devised for detecting cylinder pressure through special ports provided in the cylinder head for laboratory experimentation. The latter devices, of course, require special adaptation of the engine cylinder block and are totally impractical for field and general diagnostic applications outside the laboratory.
Other transducers have been devised for siphoning off a portion of the cylinder pressure through a small capillary tube provided in a specially-designed spark plug for a conventional spark plug-fired gasoline engine. Transducers of this type develop signal distortions due to pressure lag, frictional pressure loss and gas column resistance, as reported in the 1978 SAE Journal No. 0098-2571/ 78/8605-0054.